1. Field of the Invention
This invention relates to a metal gasket for sealing a joint between a cylinder head and a cylinder block in an internal combustion engine, particularly of the multicylinder type. More particularly, it relates to a gasket comprising two elastic metal plates laid on each other and each having a plurality of holes for combustion chambers and a bead formed along the edge of each hole, and a grommet attached to one of those plates.
2. Description of the Related Art
A reduction in weight, as well as an increase in output and an elevation in working temperature, are desireable characteristics of an internal combustion engine, and has been realized by employing aluminum, instead of cast iron, for making both a cylinder head and a cylinder block, or at least the cylinder head. Because aluminum has less rigidity, however, the cylinder head and block which are made of aluminum are likely to be lower in rigidity and easily displaceable from each other when the engine is placed in operation.
The joint between the cylinder head and the block of an internal combustion engine is sealed by a metal gasket having a bead forming an elastic seal between the deck surfaces of the cylinder head and block when fastening devices, such as bolts, are tightened for holding the cylinder head securely to the cylinder block with the gasket disposed therebetween. The tightening force applied to the bolts, however, tends to cause the cylinder head to bend, thereby creating an undesirably large clearance and distortion of material, particularly around the combustion chamber. The excess clearance and distortion cause combustion gases to leak and spoil, causing or damage to the gasket, while the substances deposited on the gasket from those gases weaken its sealing ability. The pressure of the combustion gases and the heat of the engine cause changes in the width of the clearance between the deck surfaces around the combustion chamber, and as a result, the failure of the bead on the gasket and a reduction in the tightening force of the bolts bring about a reduction in the sealing effect of the gasket.
Under these circumstances, there is known a metal gasket which comprises two elastic metal plates each having a bead formed along the edge of each bore, a grommet disposed between the elastic metal plates and close to a cylinder head and an intermediate plate disposed therebetween and close to a cylinder block, the grommet having an inner edge folded toward the cylinder block and over the inner edge of the intermediate plate, so that any clearance between the deck surfaces around the combustion chamber may be made up to prevent any reduction in function of the bead, while the elastic metal plates have a uniform spring force along the beads, so that the gasket may have improved sealing ability and durability, as disclosed in Japanese Patent Application KOKAI No. Sho 64-73157 (hereinafter referred to as "Prior Art 1"). There is also known a metal gasket which comprises two elastic metal plates each having a plurality of adjoining holes for combustion chambers and a bead formed along the edges of those holes, an intermediate plate disposed therebetween, and a grommet folded toward a cylinder head for holding one of the elastic metal plates which faces a cylinder block, and the intermediate plate having a bead formed along the bead on the elastic metal plate add projecting toward the cylinder head, as disclosed in Japanese Utility Model Application KOKAI No. Hei 4-34562 ("Prior Art 2").
Both of the prior art gaskets are, however, an assembly of four plates, and are, therefore, more expensive than any gasket formed by three plates, from the standpoint of material and manufacturing costs. Moreover, the former prior art gasket has a problem caused by the inner edge of the grommet folding toward the cylinder block for holding the inner edge of the intermediate plate. When the gasket is compressed to collapse between the cylinder head and block, the folded portion of the grommet acts as a shim for the bead on one of the elastic metal plates. As a result, the elastic metal plate facing the cylinder head has more stress than that of the plate facing the cylinder block, and the grommet has a large bending stress and a large amount of stress which tend to cause cracking and eventual failure of the grommet and the bead on the plate facing the cylinder head.
The gasket according to the latter prior art discussed above 2 is likely to lose stability under load, and suffer from a lowering of its sealing property and the cracking of its beads, since the bead on the intermediate plate contacts at its top the bead on the elastic metal plate facing the cylinder head.
The low sealing property of the known gasket, whether according either of the prior art gaskets discussed above, and the failure and cracking of their beads are apparently due to the facts that the beads are of the same construction between the bores of the cylinder head and block as around the bores, despite the presence of a difference in rigidity, and that the gaskets cannot adapt themselves to the distortion of the cylinder head and block which results from the difference in rigidity.